[PATCH v3 0/6] ext4: port direct IO to iomap infrastructure

Matthew Bobrowski
2019-09-12T11:03:40Z

This patch series ports the ext4 direct IO paths to make use of the
iomap infrastructure. The legacy buffer_head based direct IO paths
have subsequently been removed as they're now no longer in use. The
result of this change is that the direct IO implementation is much
cleaner and keeps the code isolated from the buffer_head internals. In
addition to this, a slight performance boost could be expected while
using O_SYNC | O_DIRECT IO.

The changes have been tested using xfstests in both DAX and non-DAX
modes using various filesystem configurations i.e. 4k, dioread_nolock.

Changes since v2:

- Simplified ext4_write_checks() by using file_modified() as oppose
to calling file_remove_privs() and file_update_time() explicitly.

- Other minor cleanups that have been suggested by Ritesh Harjani in
the previous round of feedback.

In the original patch series, there was a relatively lengthy
discussion around the merging of unwritten extents. The buffer_head
direct IO implementation made use of inode and end_io flags to track
whether an unwritten extent is eligible to be merged, or not. We don't
make use of these flags in the new direct IO iomap implementation,
effectively making the extent merging checks that make use these flags
redundant. However, it appears that even if additional merges and
splits are performed, it isn't deemed problematic as such and that's
due to the way that the filesystem now accommdates for unexpected
extent splits. The only real concern is the potential wasted
performance due to the unnecessary merge and split performed under
specific workloads. The full discussion that goes through these
details starts from here:
https://www.spinics.net/lists/linux-ext4/msg67173.html.

Thank you to all that took the time to review and provide constructive
feedback for previous patch series, highly appreciated.

Matthew Bobrowski (6):
ext4: introduce direct IO read path using iomap infrastructure
ext4: move inode extension/truncate code out from ext4_iomap_end()
iomap: split size and error for iomap_dio_rw ->end_io
ext4: reorder map.m_flags checks in ext4_iomap_begin()
ext4: introduce direct IO write path using iomap infrastructure
ext4: cleanup legacy buffer_head direct IO code

fs/ext4/ext4.h | 3 -
fs/ext4/extents.c | 11 +-
fs/ext4/file.c | 364 +++++++++++++++++++++++------
fs/ext4/inode.c | 516 +++++-------------------------------------
fs/iomap/direct-io.c | 9 +-
fs/xfs/xfs_file.c | 8 +-
include/linux/iomap.h | 4 +-
7 files changed, 363 insertions(+), 552 deletions(-)

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2.20.1